The present invention relates to recessed lighting and, more particularly, to thermal management for recessed lighting.
Recessed downlights are a large category of lighting in the United States. Recessed downlights provide a fixture that is mounted in the ceiling such that the light source is recessed above the plane of the ceiling. A wide variety of these recessed downlight fixtures are used in both residential and commercial applications. The fixtures typically utilize an incandescent Parabolic Aluminized Reflector (PAR) lamp as the light source, although recessed downlight fixtures specifically designed for compact fluorescents are also utilized.
Recently solid state lighting systems have been developed for general illumination applications. The design of a solid state lighting system for general illumination typically involves designing optical, power and thermal management systems in order to provide a particular level of performance with respect to lumen output, power requirements and junction temperature of Light Emitting Diode (LED) light sources. The junction temperature of LEDs may be important as it may be a contributing factor in the lifetime of the LEDs. In particular, if the junction temperature exceeds the recommended junction temperature of the manufacturer, then the LEDs will typically not achieve the lifetime rated by the manufacturer. Furthermore, as the operating temperature of LEDs changes, the current through the LEDs may change. For this and other reasons, changes in operating temperature can result in color shifts in the resulting light output. Maintaining a stable operating temperature may, therefore, also benefit in maintaining stable color output of a solid state light source.
Thermal management for solid state lighting systems has generally fallen into two categories: passive systems and active systems. These systems have typically been integral to the lighting device. Thus, for example, the LR6 recessed downlight from Cree LED Lighting Solutions of Morrisville, N.C., utilizes a passive system that incorporates a heat sink that is exposed to the room in which the LR6 is mounted. Thus, the LR6 provides not only the light source but also the trim for a recessed fixture in which the LR6 is mounted. By exposing the heat sink to the room the LR6 benefits from any air currents that break the boundary layer between the heat sink and the air in the room. Breaking the boundary layer between a heat sink and its environment can increase the efficacy of the heat sink, thereby lowering the junction temperature of the LEDs.
Active thermal management for solid state lighting systems has also been utilized. For example, U.S. Pat. No. 7,144,135 entitled “LED Lamp Heat Sink” describes an LED lamp that includes a fan that moves air over a heat sink. Additionally, LED downlights with integral synthetic jet cooling systems have also been announced by Nuventix and Philips. See nuventix.com/news/Nuventix-Announces-SynJet-Fanless-Air-Cooler-for-Philips-Fortimo-LED-Downlight-Module date Apr. 7, 2008 on the World Wide Web.